The present invention relates generally to apparatus for removing material from the bottom of a water filled reservoir and more particularly to a dredge which uses a novel combination of both mechanical clamshell and hydraulic dredge technology.
It will be appreciated by those skilled in the art that there are two primary types of dredges currently in use. A mechanical dredge uses a clamshell or dragline positioned from a floating base to dig and remove solid material from beneath the water. There are a number of difficulties created by the use of conventional clamshell and dragline dredges. First, the clamshell is typically suspended and allowed to free fall through the water so that it will impact the bottom with sufficient force to penetrate and dig into the material to be removed. This disturbs the surrounding sediments, causing re-suspension into the water of particles and contaminants. Similar disturbances are produced by pulling a dragline randomly through the sediments. The accuracy of positioning of the clamshell or dragline during these operations is less than optimal as well. Second, the clamshell or dragline bucket must be dumped either immediately behind the dredge, or in a scow or dump truck, for transportation to a disposal site. The cost and difficulty of this type of transportation limits the application of the clamshell dredge.
To address some of the drawbacks of mechanical dredges, many have begun using the hydraulic dredge which uses a cutting device to loosen material from the bottom and a centrifugal pump to transport the dredged material in the form of a slurry. Typically, the water content of the slurry ranges from seventy percent (70%) to one hundred percent (100%), with pipe line velocities maintained at between seven (7) feet to fifteen (15) feet per second in order to achieve adequate particulate suspension. Because of the high water content of the slurry and high pump velocities, disposal of the slurry is a problem, particularly if it contains hazardous waste material. A common method of dealing with the disposal problem is to pump the slurry into a lagoon or settling basin and let the excess water run-off. This may or may not be permitted by regulatory authorities because the water may contain hazardous constituents. Conventional hydraulic dredges are sometimes inappropriate for use with materials such as paper mill sludge that, when disturbed, change characteristics to the point that they cannot effectively settle or de-water in the disposal lagoon.
Finally, prior art dredging systems do not easily accommodate either visual inspection of the removed sediment or pre-pumping processing of the sediment by screening, grinding and the like.
U.S. Pat. No. 3,352,035 issued to F. Joyce on Nov. 14, 1967, discloses one type of hydraulic dredge. Unfortunately, this type of dredge is only useable with fine and soft material because of the limitation created by the dredge hose.
U.S. Pat. No. 3,973,575 issued to A. Sullivan, et al on Aug. 10, 1976, discloses another type of hydraulic dredge which uses a shovel which is dragged along the ocean bottom. This type of cutter creates a large amount of re-suspension thereby decreasing the efficiency of the hydraulic dredge. Further, it is difficult to control the area of excavation with a dragline.
U.S. Pat. No. 4,052,800 issued to A. Fuhrboter, et al on Oct. 11, 1977, also discloses a shovel which is towed by one boat and a hydraulic dredge which is towed by another boat. As stated above, the problem with this apparatus is that the centrifugal pump has a low solid to water content thereby decreasing the efficiency of the hydraulic dredge. Further, the dragline causes significant re-suspension of materials. Also, the area of excavation is difficult to control.
U.S. Pat. No. 4,357,764 issued to P. Lemercier on Nov. 9, 1982, discloses a vehicle which uses a vertical auger system. Unfortunately, this device has very limited application because the vehicle will quickly fill up and will not be able to carry any more dredged material. Further, the device is very complicated in that the vehicle must drive under water.
U.S. Pat. No. 4,680,879 issued to C. Hill, et al on Jul. 21, 1987, discloses a hydraulic dredge having a cutter head. As stated repeatedly throughout, the standard hydraulic dredge pulls in water at a low solid to water ratio thereby decreasing the efficiency of the dredge.
U.S. Pat. No. 5,042,178 issued to B. Dutra on Aug. 27, 1991, discloses a clamshell dredge and hopper which uses water jets for the reduction of relatively dry solids into a high density slurry for the purpose of filling a barge. The clamshell is suspended from a line thereby making it difficult to control excavation.
What is needed, then, is a dredge apparatus which can move dredged material a substantial distance away from the dredge without a high concentration of water and at low velocities. This apparatus also should allow for precise control of the removal process underneath the water and without significant re-suspension of particulate at the dredging location. This device is presently lacking in the prior art.